Conventionally, as disclosed in Patent Document 1, as a superfinishing method, there is known a method of superfinishing an outer peripheral surface of an annular member, which is an object to be machined, by rotating the annular member around its center axis, in a state in which a grinding stone is brought into pressure contact with the outer peripheral surface of the annular member, and simultaneously oscillating the grinding stone in a direction parallel with the center axis of the annular member along a plane touching a generatrix of the outer peripheral surface of the annular member.
Such a superfinishing method is often used for superfinishing an element of a roller bearing, including rollers or a raceway surface of an inner ring or an outer ring. For example, in Patent Document 2, the superfinishing is performed on the raceway surface of the outer ring of a tapered roller bearing by the same method as the above-described method. More specifically, as illustrated in FIG. 6A, the superfinishing is performed by rotating an outer ring 110 of the tapered roller bearing around a center axis O′, and simultaneously, vertically pressing a distal end surface of the grinding stone 103 against a tapered raceway surface 114 formed on an outer peripheral surface of the outer ring 110 and linearly reciprocating the grinding stone in a direction of the center axis O′.
Conditions in which the distal end surface 104 of the grinding stone 103 and a logarithmically crowned surface of the raceway surface 114 of the outer ring 110 contact each other in machining according to the conventional superfinishing method are illustrated in FIG. 6. In FIG. 6A, reference signs a, b, c indicate one end, an intermediate portion, and the other end in the linearly reciprocating movement of the grinding stone 103, respectively. Reference signs I, II, III indicate imaginary planes passing through one side portion, an intermediate portion and the other side portion of the grinding stone 103 in a circumferential direction, respectively. Further, in FIG. 6B, the portion, indicated by a double wavy line, on the distal end surface 104 of the grinding stone 103 represents a portion brought into contact with the raceway surface 114 of the outer ring 110. In the intermediate portion II of the grinding stone 103 in the circumferential direction, the entire surface of the raceway surface 114 of the outer ring 110 always abuts against the distal end surface 104 of the grinding stone 103 at all positions a, b, c when the grinding stone 103 reciprocates linearly. Accordingly, the raceway surface 114 of the outer ring 110 can be superfinished uniformly in an axial direction.
When the roller comes into contact with the raceway surface in the roller bearing, an excessive pressure (so-called edge load) is generally generated at the edge portion of a contact region. To avoid the edge load, the roller or the raceway surface of the inner ring or the outer ring is formed with a slight bulge referred to as a crowning represented by a logarithmic function (hereinafter, the surface provided with the crowning is referred to as a logarithmically crowned surface).
When the raceway surface made of the logarithmically crowned surface is subjected to the superfinishing, the superfinishing method disclosed in Patent Document 1 or 2 is employed, a grinding amount of the logarithmically crowned surface is not uniform in the axial direction of a rolling contact surface, so that a finished surface having the uniform surface roughness may not be obtained. More specifically, as illustrated in FIG. 7, if the raceway surface 114 of the outer ring 110 is made of the logarithmically crowned surface, when the grinding stone 103 is first displaced at the position a, a first crowning drop portion 116 at one side in the axial direction does not abut against the distal end surface 104 of the grinding stone 103. Further, when the grinding stone 103 is displaced at the position b, the first and second crowning drop portions 116, 117 at both sides in the axial direction do not abut against the distal end surface 104 of the grinding stone 103. In addition, when the grinding stone 103 is displaced at the position c, the distal end surface does not abut against the second crowning drop portion 117. That is, when the grinding stone reciprocates linearly, the distal end surface 104 of the grinding stone 103 does not abut against all the crowning center portion 115, the first crowning drop portion 116 and the second crowning drop portion 117 of the raceway surface 114 at any of the positions a, b, c. Therefore, as illustrated in FIG. 8, since the surface roughness of the crowning center portion 115 and the first and second crowning drop portions 116, 117 is varied, the surface roughness of the raceway surface 114 may not be uniform.
Accordingly, according to the superfinishing method disclosed in Patent Document 3, the finishing is performed by traversing the grinding stone in parallel with the crowned surface so that a swing center axis is always maintained in a normal direction of the crowned surface formed on an oscillation surface, thereby preventing a collapse of its shape.